Method for synthesizing lactide by means of catalysis of lactid acid

Abstract

The present invention relates to a method for the catalytic synthesis of lactide from lactic acid. The method relates to the synthesis of lactide from lactic acid under the catalysis of a zinc oxide nanoparticle aqueous dispersion as a catalyst. The present invention has four technical characteristics: I. the zinc oxide nanoparticle aqueous dispersion catalyst has a sufficient surface area, and the size of nanoparticles is merely 30-40 nm, providing a sufficient contact area between the substrate (lactic acid) and the catalyst; II. the new catalyst has a milder catalytic effect on polymerization, allowing the molecular weight distribution of a prepolymer within a range of 400-1500 g / mol, which is advantageous for depolymerization to proceed; III. the new catalyst is stable, thus avoiding oxidation or carbonization in a high temperature reaction; and IV. the new catalyst has a low toxicity and a small threat to human health.

Description

TECHNICAL FIELD[0001]The present invention relates to the technical field of the preparation of biodegradable polymers, and more specifically relates to a method for the catalytic synthesis of lactide from lactic acid.BACKGROUND ART[0002]The techniques for the synthesis of lactide from lactic acid has been developed for several decades and the synthesis method thereof has been described in numerous patents, such as U.S. Pat. Nos. 5,053,522 A, 5,247,058 A, 5,357,035 A, 6,005,067 A, 6,277,951 B1, 6,326,458 B1, 5,274,127 A, US 20050222379 A1, US 20120302724 A1, US 20110155557 A1, CN 1951933 A, CN 1594313 A, CN 1488628 A, and WO 2010105143 A2. At the present stage, methods mainly used in industrial productions relate to heating at high temperatures and using tin-based chemicals as catalysts (such as tin (II) 2-ethylhexanoate and tin chloride) in a vacuum environment. In order to avoid the oxidation of the reactants under high temperature conditions (≥180° C.), the chemical reaction is u...

AI Technical Summary

Benefits of technology

[0011]In the method for the catalytic synthesis of lactide from lactic acid as provided by the present invention, said zinc oxide nanoparticle aqueous dispersion is a dispersion of zinc oxide nanoparticles in water, and the particle size of the zinc oxide nanoparticles is 30-40 nm, and the mass percentage of the zinc oxide nanoparticles is 20%. The present invention has four technical characteristics: I. the zinc oxide nanoparticle aqueous dispersion catalyst has a sufficient surface area, and the size of nanoparticles is merely 30-40 nm, providing a sufficient contact area between the substrate (lactic acid) and the catalyst; II. the new catalyst has a mild catalytic effect on polymerization, allowing the molecular weight distribution of a prepolymer within a range of 400-1500 g / mol, which is advantageous for depolymerization to proceed; III. the new catalyst is stable, thus avoiding oxidation or carbonization in a high temperature reaction; and IV. the new catalyst has a low toxicity and poses a small threat to human health.

[0019]Implementing the present invention results in the following beneficial effects:

[0020]1) Higher synthesis efficiency. The zinc oxide nanoparticle aqueous dispersion as a novel catalyst has a larger surface area than conventional solid or liquid catalysts. Therefore, the catalyst can be in better contact with lactic acid, improving the catalytic efficiency. Moreover, in the polymerization reaction and depolymerization reaction, zinc oxide nanoparticles can promote the equilibrium of the reaction to the depolymerization reaction, thereby increasing the synthesis yield and increasing the production yield to about 90%;

[0021]2) Lower energy consumption. The high catalytic efficiency of the zinc oxide nanoparticles facilitates the depolymerization and distillation purification, which can result in obtaining lactide by separation at a lower temperature and within a shorter time (170-220° C., 3-5 hours), reducing energy consumption, as compared with traditional methods (180-250° C., 8-10 hours). Furthermore, the lower temperature can avoid the racemization and carbonization of the prepolymer and lactide, improving the optical purity of the product;

[0022]3) Lower safety risk. The catalyst in this new method is less toxic than conventional tin-based catalysts. Besides, the risk of inhalation of conventional solid particles of heavy metal catalysts into human bodies can be avoided by the dispersion form.

Problems solved by technology

Conventional tin-based catalysts are more conducive to polymerization rather than depolymerization, resulting in increased difficulty in the synthesis and separation of lactide.

In addition, the low catalytic efficiency of other metal compounds for polymerization and degradation leads to a decrease in the production yield of lactide (<70%).

Therefore, traditional lactide synthesis methods have a low production yield and need to be further optimized and improved.

Images